I wanted to have this posted as this comes up from time to time on patch days or when you are attempting to patch during any point. This should help you for troubleshooting this error.
When attempting to patch, you get the error "Failed to apply patch. Please try again later. If this problem persists you may need to reinstall or contact technical support"
This error message only appears when you are trying to log into WoW directly (bypassing the launcher), and there is a patch that needs to be done.
Make sure you are using the World of Warcraft Launcher.app to patch and start the game. This should be located in /Applications/World of Warcraft
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